1. Field of Invention
The present invention pertains to the field of video communication. More particularly, this invention relates to video communication using multiple encoded streams.
2. Art Background
A wide variety of video communication systems including digital television systems, Internet video systems, and wireless video systems commonly employ communication links to carry digital video. Typically in such systems, a digital video originates in a sender and is transferred to one or more receivers via one or more communication links. In a digital television system, for example, a broadcast station as a sender usually transmits digital video to one or more digital television receivers via over-the-air or cable communication links. In an Internet video system, as another example, a web server typically acts as a sender that transfers digital video to one or more web client receivers via the Internet.
Digital video in such systems is typically arranged as a series of video frames. The video frames usually occur at high enough frame rate to enable a viewer to perceive full motion video when the video frames are rendered on a display. For example, a frame rate between 15 and 30 frames per second is common in video communication systems. Each video frame usually carries an array of pixel data. The relatively high amount of pixel data contained in each video frame combined with a frame rate high enough to provide full motion video would typically impose a relatively high bandwidth consumption on a communication link.
Prior video communication systems commonly employ video compression to reduce the bandwidth consumption of digital video. Typically, a sender includes an encoder that generates a series of encoded frames in response to a series of original video frames. Each receiver usually includes a decoder which reconstructs the original series of video frames from the encoded frames. The total amount of data carried in the encoded frames is usually significantly less than the total amount of data in the corresponding original video frames.
The encoded frames in prior video compression methods typically include frames which carry all of the information needed to reconstruct the corresponding original video frame. Such frames may be referred to as intra frames (I-frames). In addition, the encoded frames in prior video compression methods typically include frames for which reconstruction of the corresponding original video frame depends on a prior encoded frame from the series of encoded frames. Such frames may be referred to as predicted frames (P-frames) because they are commonly generated by an encoder using a prediction loop.
Typically, the amount of data carried in an I-frame is significantly greater than the amount of data carried in a P-frame. Therefore, bandwidth savings is usually enhanced if a greater percentage of the encoded frames are P-frames. Unfortunately, when using prediction the loss of a P-frame or I-frame during transmission typically prevents the reconstruction of the corresponding original video frame as well as the reconstruction of a sequence of subsequent P-frames before a next I-frame. The loss of a sequence of P-frames usually has negative effects on the reconstructed digital video. Such effects may result in freeze frame or the appearance of displayed artifacts. These negative effects are typically worsened in systems which use a relatively large number of P-frames between I-frames due to communication bandwidth constraints.
A video communication system is disclosed that uses multiple streams to carry digital video. The use of multiple streams ensures that errors in one or more of the multiple streams do not prevent reconstruction of remaining ones of the multiple streams. This enables an error free display of the digital video at a reduced frame rate during the loss of a subset of the streams. In addition, the multiple streams provide past and future frames that may be used to recover lost frames.
Other features and advantages of the present invention will be apparent from the detailed description that follows.